Operation
83
4
SPI is a convenient interface because it is available on many microprocessors, provides relatively high speed
communications, and uses only 4 signals; SPIClk (Clock), SPICS (chip select), SPISI (slave in), and SPISO (slave out).
Atlas utilizes standard SPI signaling and timing control for the hardware interface and implements a higher level
protocol on top of this. See
Section 3.6, “AC Characteristics”
to learn more about low-level hardware SPI signal timing,
voltage levels, etc.
Figure 4-21:
SPI
Communica-
tions Protocol
Overview
All communications to and from Atlas are in the form of a packet.
shows the overall packet format. A
falling edge of the chip select begins the packet, and a rising edge of the chip select ends the packet. All Atlas SPI
packets are comprised of a two word header and one or more optional command words.
The first two words of the packet are called the header and are used to specify a desired motor voltage or torque along
with certain other functions such as when a trace starts and when a command update should occur.
As shown in
each SPI word sent from the external controller to Atlas results in a return word sent from
Atlas to the external controller. In fact at a signal level, each outgoing bit is sent simultaneously with each incoming
bit, providing full duplex communications.
The external controller must receive and process data words sent to it by Atlas. These return words, depending on the
context, contain transmission integrity information, status bits, or other useful information.
4.15.1 Sending a Voltage or Torque Output Value
Figure 4-22:
Sending a
Voltage or
Torque Output
Value
Generally the most frequently used header transaction is an instantaneous voltage or torque output request for output
by the drive. If Atlas is used as part of a higher level velocity or position controller, then these values are continuously
sent to Atlas at the servo sampling speed of the main motion controller, typically between 1 kHz and 10 kHz.
shows the overall format of the header words when voltage or torque commands are being sent.
Depending on the motor type and command mode being used the data words will be loaded one of several different
ways.
For a complete description of these fields, along with many other details of Atlas SPI operation, refer to the
Atlas
Digital Amplifier Complete Technical Reference.
Atlas® Digital Amplifier Complete Technical Reference
Controller word 0
x
t
u
0
Header data 1
Atlas word 0
SPI Status word
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2
1
0
Controller word 1
Header data 2
Atlas word 1
Atlas checksum
Controller checksum
15
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2
1
0
Controller word 2-N
Optional command
Atlas word 2-N
Optional command response
15
14
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2
1
0
Controller word 0
0
t
u
0
Data 1
Atlas word 0
SPI Status word
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2
1
0
Controller word 1
Data 2
Atlas word 1
Atlas checksum
Controller checksum
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8
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3
2
1
0
